This article explores various functional aspects of hydraulic free piston engine (FPE) enabled by action motion control. Given the potential for high efficiency and flexibility, the FPE is well suited for mobile applications such as on-road vehicles and off-road heavy machinery. The advantage of the active motion controller lies in its ability to precisely track and shape the piston trajectory. FPE has a great potential for energy saving and emission control, but its reliable operation is limited by the complex dynamic coupling among the engine subsystems and the lack of the crankshaft. This inherent technical barrier for FPE could be overcome by active control with today’s sensing, actuation and computing technologies. A prototype hydraulic FPE is used to demonstrate the capabilities of active piston motion control. Experimental results demonstrate the feasibility and promise of the technology. Engine power control will be combined with piston motion control in the future to achieve a wider range of engine operation and higher engine efficiency.

## Article

Compression ratio is an important factor that determines engine efficiency, and the flexibility of an internal combustion engine (ICE) will be increased significantly if its compression ratio can be varied in real-time. Different variable compression ratio mechanisms have been proposed by modifying the crankshaft design. The free piston engine (FPE), however, offers the ultimate flexibility for variable compression ratio control by eliminating the crankshaft. The merit of this setup lies in its simple design with few moving parts, which results in a compact engine with low maintenance costs and reduced frictional losses.

Pescaraef0011 is credited for the invention of the FPE in the 1920s. Reliable operation of the FPE could not be achieved due to the absence of proper sensing and control technologies at that time. Recently, researchers have re-discovered the huge potential of FPE with the advancements in sensing, actuation and computing technologies. A comprehensive review of recent work on FPE can be found in Mikalsen and Roskilly’s “A review of free-piston engine history and applications,” published in Applied Thermal Engineeringef0022. The FPE discussed in this article is an opposed-piston opposed-cylinder (OPOC), two-stroke engine which offers the highest power density and scavenging efficiency among the FPE architectures. Figure 1 shows both a photograph and a schematic of the engine. Combustion in the right cylinder pushes the inner piston to the left and outer piston to the right, which compresses gas in the left cylinder. The two cylinders fire alternately while high-pressure fluid is being pumped out via the piston motion. In other words, the engine converts chemical energy into fluid power. This design eliminates the mechanical linkages between each engine unit and hence drastically increases the modularity of the system. Given its potential for high efficiency and flexibility, the FPE is well suited for mobile applications such as on-road vehicles and off-road heavy machinery.

The major technical barrier for FPE is the large cycle-to-cycle variation, especially during transient operations. Based on simulation studiesef0033,ef0044, the engine in Fig. 1 is found to be unstable at various loading conditions. This is due to the fact that the engine operation is relatively complex in a way that the dynamics of the subsystems are heavily coupled as shown in Figure 2. Specifically, the piston motion affects the hydraulic dynamics, the gas mixing, and the combustion process. In return, the combustion pressure and the hydraulic pressure further determine the piston dynamics. The dynamic couplings are revealed in Eq. 1 as well.

$x˙1M=Pleftx,x˙⋅Ap−Prightx,x˙⋅Ap−Ffx˙+Fhydx,x˙$

where x is the piston displacement, M is the piston mass, Ap is the piston area, Pleft and Pright are the left and right combustion chamber pressures, Ffand Fhyd are the forces from friction and hydraulic chamber, respectively.

The intrinsic feedback nature of the engine implies that active control is essential to ensure stable operation. Therefore a robust and precise piston motion controller is designedef0055. The controller acts as a virtual crankshaft that guides the piston to follow a reference trajectory via the hydraulic actuator (servo valve) by utilizing energy from the storage element. Given the periodic nature of the piston motion, the active controller employed here is of the robust repetitive type, which is capable of tracking any periodic reference signal with fast convergence rate and small steady state error.

The advantage of the active motion controller lies in its ability to precisely track and shape the piston trajectory. Specifically, the reference trajectory of the virtual crankshaft can be altered digitally, in real-time, to achieve a wide range of piston motions, and thus obtain maximum engine efficiency with respect to various operating points. Figure 3 shows the engine motoring tests results. The virtual crankshaft is able to produce stable and repeatable motoring with precise piston motion tracking.

shows the PV diagrams from preliminary combustion tests. Even though cycle-to-cycle variation can be observed in the graph, stable engine operation is still achieved, which demonstrates the effectiveness of the active motion controller.

FPE has great potential for energy saving and emission control, but its reliable operation is limited by the complex dynamic coupling among the engine subsystems and the lack of the crankshaft. This inherent technical barrier for FPE could be overcome by active control with today’s sensing, actuation and computing technologies. A prototype hydraulic FPE is used to demonstrate the capabilities of active piston motion control. Experimental results demonstrate the feasibility and promise of the technology. Engine power control will be combined with piston motion control in the future, to achieve a wider range of engine operation and higher engine efficiency.

## Acknowledgments

The authors would like to thank the NSF Center for Compact and Efficient Fluid Power (CCEFP) for the financial support (EEC-0540834) and Ford Motor Company for donating the hydraulic free piston engine.

## References

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